text:
Why are commercial airplanes painted at all?
While there was debate over why airplanes  are painted white and a question about how much a new paint job costed, what hasn't been asked is why airplanes are painted at all.
I remember one airline company sent out a press release in the 1980s or 1990s that keeping its planes unpainted (i.e, a nice silver) saved several hundred gallons of paint as well as considerable cost savings because the plane was also lighter and used less fuel.
If there are considerable positives to keeping planes unpainted, why do airline companies bother painting them?
This is prompted by a recent flight I had on an US Air airplane that had just been repainted in the American Airlines livery. The captain joked that the paint was still so new that we shouldn't try touching it. Both companies are famous for always being in severe debt -- so saving every penny should have meant going for the 'naked' look.
Related: A quora question on how much money would be saved if airlines stopped painting their liveries and a Boeing white paper on painted vs unpainted

First two launches of the space shuttle contained also some hundred kilograms of unneeded paint.

A new twist on my question: http://www.theverge.com/2015/4/4/8344437/nasa-tests-bug-repellant-coating-for-airplanes

You've accepted an incorrect answer. The Boeing document referenced in the answer recommends to wash all planes, whether painted or polished, in order to reduce drag. It states that both painted and polished aircraft are aerodynamically clean, and refutes the claim that paint reduces drag.

@koyovis - write this an answer giving references to your argument that no paint is better. You might also want to refute the maintenance argument that there's a limit to polishing metal.

Give this a read, quite interesting: https://www.scienceabc.com/eyeopeners/why-airplanes-are-usually-white-in-color.html

I remembered hearing that paint actually reduces the drag of the plane. Searching a bit I found this Boeing document (archived here) describing it.
So the painting reduces the fuel costs, instead of increasing them. Another interesting thing is, that planes are regularly repainted to keep the drag low. Of course there is the billboard effect too.
From the report:

In response to numerous questions raised by
Boeing customers regarding the efficacy of
surface coatings to reduce drag, Boeing has
investigated some possible airflow physics
explanations. Possible explanations postulated by Boeing aerodynamicists include:

Increased regions of laminar flow due to reduced surface roughness.
Reduction in surface roughness resulting in lower skin friction drag, when flow is turbulent.
Reduction in dirt and/or insect adhesion resulting in reduced roughness and hence reduced skin friction drag.

a proper polishing would also keep the plane low drag but polishing aluminum properly is harder than applying a coat of paint

Polishing is a form of abrasion, which therefore removes material. How many times can an aircraft's skin be polished before the probability of failure increases unacceptably?

polishing won't ever fill in gaps between plates, paint can provide a smooth surface by filling those gaps.

I remember reading the story in American Airlines in-flight magazine about why they polished instead of painted, and it was all about the weight savings.  Across the fleet it amounted to millions of dollars of savings in fuel.  Apparently Cathay Pacific used the same justification for their freighter fleet, where polishing just 14 747s was predicted to save them aboutHK$2,800,000 in 2006  http://www.pprune.org/tech-log/444643-all-metal-livery-cargo-planes.html#post6286246

A more complete analysis by Boeing:  http://www.boeing.com/commercial/aeromagazine/aero_05/fo/fo01/index.html

You answer is incorrect. The document states that both painted and polished surfaces are aerodynamically smooth, and recommends to wash aircraft regularly.

I would wax the plane! Polish is for scratches and rust film, wax for keeping the surface in good shape. Meguiars is really good, while their products look like a rainbow for a european (too much "blingbling").

Also, a light-colored paint more easily shows fluid leaks and external damage on the aircraft's surface.

Quite apart from the paint preventing corrosion, which puts the lie to the idea that AA flies unpainted aircraft (they don't, they use a clear paint on metallic parts and metallic paint on non-metallic parts), there's the economic factor.
Those aircraft are flying billboards for their owners. They are a major part of brand recognition.
That's why they're repainted or at the very least have stickers applied when rented/leased out to other operators.

Well, American definitely doesn't fly unpainted aircraft now!

@Sean AA never flew them unpainted. They were painted in a transparent coating only, then later metallic silver paint over non-metallic parts and a clear coat over the rest. And now indeed they're dull boring grey.

Painted aircraft can be kept looking clean with a relatively quick wash, while unpainted metal requires comparatively more effort and time (thus, money) to be kept shiny. The fuel savings may simply be more than offset by the maintenance fees.

I think more importantly it also helps the ground crews and the air traffic control tower responsible for ground traffic.  When looking at a line up of planes from a distance, you can eliminate a majority of planes right away because they are either the wrong airline or wrong type.  Now you only have to compare the tail numbers of a couple aircraft.
Several years back I was getting a tour from a friend of orlando's control tower, and as you can imagine it is a pretty busy place especially up top where they are relying on their eyes to organize the traffic on the ground.  When tracking an aircraft that's taxing, they have two pieces of information, tail number and airline.  So if you have a lineup that goes AA, United, UPS, United, Frontier for instance, you don't even have to break out the binoculars to know what their tail numbers are except to maybe to differentiate between the two united flights.  Speeds up the process and also cuts down on careless error.

ground crews would respond to a specific parking spot, no reason to look at each aircraft in a long line "nope, wrong owner. nope, wrong type. wrong owner. wrong owner. hmm, could be the one. nope, wrong serial. wrong owner. ah, here we are!" isn't how they operate :)

@jwenting ground crews are just a second benefit, air control is the primary because airplanes easily get out of order during peak times. On a side note,  if you think that the correct aircraft is in the correct spot and the information is correct relayed to the ground crews all the time then i definitely need to start flying out of your airport! :P  Not to mention all the airports that have an overflow lot where airlines line up like a car parking lot with very little indication or reasoning, they just need to get the airlines off the taxiways.

This is an interesting benefit, but I'd guess it's a secondary benefit at most. At the many of the busiest fields (including the busiest one in the world by far) the vast majority of the traffic is from a single airline with a single livery on almost every aircraft at the field. This helps if you have AA, United, UPS lineup, but not so much when you have a Delta, Delta, Delta, Delta, Delta, Delta lineup (or FedEx, FedEx, FedEx, FedEx in Memphis.) It would be useful at Orlando, but not as much at the hubs.

Why are aircraft painted at all?

Because of the flying billboard function. Marketing determines the colour scheme, not techical reasons such as weight savings (which is true) or drag reduction from paint (which is false).
Because of protection. The paint protects against small scratches from sand etc which may start fatigue cracks. If the polished skin is re-buffed 3 times a year it is closely scrutinised - but preventing is better than curing. Polishing removes a very thin layer of aluminium oxide and I could find no references that the amount of removed material would be a problem during the lifetime of an aircraft.

On a 737-800, the coat of paint weighs about 70 kg, on a B-777 about 200 kg. The cost increase due to higher maintenance requirement is about 60 kUSD in 1998 dollars or 85,000 dollars/year now, for an airline with 100 B737s this equates to higher maintenance cost of 8.5 mUSD/year. This is offset by the reduced fuel costs from the weight savings, depending on air miles.
Corrosion prevention is an often quoted reason for the coat of paint. However there is a specific type of corrosion occurring only on painted aircraft.
From a Boeing document:

Polished surfaces are protected from corrosion by regular buffing
  after washing. Painting protects against oxidation, salts, and jet
  fuel spills. However, unrepaired chips and cracks in paint collect
  dirt and moisture and so may become corrosion sites. Painted surfaces
  are also susceptible to filiform corrosion, or worm corrosion, which
  begins between metallic surfaces and paint and erodes both. It creates
  hydrogen and lifts up the paint layer as it travels across the
  surface.

Another Boeing document addresses claims that paint may reduce drag. It effectively refutes these claims, stating that both painted and polished aeroplanes are aerodynamically clean. It does recommend to keep them that way, by washing all planes at least three times a year, due to accumulated dirt and insects which do cause increase of drag. Something that was found by the regular buffing of polished aircraft, I believe that in the past polished aircraft were found to have lower operating cost which was first attributed to weight savings but later turned out to be due to less insect residue.
All in all, aircraft are painted for good reason. And sometimes they are not painted, also for good reason.

Actually, Airbus requires their planes to be painted, for protection.

Yes that makes sense. An insurance policy for if the required increase of maintenance is not followed.

authors: Bob Jarvis - Слава Україні


text:
Is it illegal to fly a Cessna 172S under day VFR with an inoperative vacuum gauge?
Let's say my vacuum gauge is not working on my C172S and I want to go day VFR flying.  There is no MEL.  14 CFR 91.213 details procedures for inoperative instruments, and I think the following paragraph applies to my operation, so let me go through these one by one:

(d) Except for operations conducted in accordance with paragraph (a) or (c) of this section, a person may takeoff an aircraft in operations
  conducted under this part with inoperative instruments and equipment
  without an approved Minimum Equipment List provided -
...
(2) The
  inoperative instruments and equipment are not -
(i) Part of the VFR-day type certification instruments and equipment
  prescribed in the applicable airworthiness regulations under which the
  aircraft was type certificated; (N/A, vacuum gauge not part of TOMATO FLAMES)
(ii) Indicated as required on the aircraft's equipment list, or on the
  Kinds of Operations Equipment List for the kind of flight operation
  being conducted; (not sure here*)
(iii) Required by § 91.205 or any other rule of this part for the
  specific kind of flight operation being conducted; (N/A, not conducting flight at night or IFR)
(iv) Required to be operational by an airworthiness directive; (N/A, let's assume no applibale ADs)

*This is interesting language: "Indicated as required on the aircraft's equipment list".  AFAIK, to determine if the vacuum gauge fits this description, I should consult the equipment list in the POH, which reads:

Ok, so I now assume any instrument with a number ending in "-R" is "required" for the purposes of complying with the above regulation.  Locating the vacuum gauge in the equipment list, I see it indeed has a suffix "-R", indicating it is a "required item for FAA certification":

Thus I am led to conclude that the vacuum gauge is "indicated as required on the aircraft's equipment list", and hence continuing my flight would be in violation of 14 CFR 91.213.
Is my reading of the regulation and my interpretation of "required item" accurate here?

Closely related, perhaps a dupe?

I don't have time to write it up now, but my initial answer would be that the vac/amp gauge is required, for day VFR flight, and accordingly one is needed.  A question is what is the process to be able to fly without the vac gauge operative, and just the ampmeter.  However, I would like to point out that in most C172 the vac pump is driven off the accessory drive, and failure of the pump could put the accessory drive at risk, and accordingly there is value in detection of a vac failure, therefore it would be imprudent to operate without the gauge.  The determination could vary between FSDOs.

Which 172S?  G1000?

What does "TOMATO FLAMES" mean? I'm assuming it doesn't refer to a pilot burning tomatoes in flight?

No, you're mistaking the equipment list required for certification of the airplane with the Minimum Equipment List, which is a list of equipment that can be inoperative when a flight is begun. Small GA airplanes often don't have a MEL, but larger airplanes that have multiple instances of equipment for redundancy, may have the equipment listed on the MEL, indicating for how long the item can be inop, and any limitations, especially if some other equip has also failed, which may reduce the safety margin too much.

P.S.: Required for certification means the Type Certificate, which is the FAA's certification that the design is per regulations, e.g. the design may need 3 fuel pumps to meet the reliability calculations so that the probability of a fuel starvation event is extremely remote. But for operational purposes the airplane may be dispatched to fly for 10 hours with one pump inop, provided the two other pumps and the fuel pressure gauge are operational.

Your reading is correct: a Cessna 172 with an inoperative vacuum gauge is not airworthy.
Type Certificate Data Sheet No. 3A12, Revision 85 shows how Cessna 172 models meet FAA airworthiness requirements. Section XII, “Model 172S, Skyhawk SP, 4 PCLM (Normal Category), 2 PCLM (Utility Category), Approved May 1, 1998,” contains

Equipment
The basic required equipment as prescribed in the applicable airworthiness regulations (see Certification Basis) must be installed in the airplane for certification …
NOTE 2: Pilot’s Operating Handbook and FAA Approved Airplane Flight Manual (POH/AFM):  part number 172SPHUS-00 (or later approved revision) is applicable to the Model 172S. The airplane must be operated according to the appropriate POH/AFM …

The Comprehensive Equipment List in Section 6 of the POH/AFM gives the Combination Vacuum Ga[u]ge/Ammeter the R suffix, meaning required.
FAR 91.213(d)(2)(ii) reads

(d) Except for operations conducted in accordance with paragraph (a) or (c) of this section, a person may takeoff an aircraft in operations conducted under this part with inoperative instruments and equipment without an approved Minimum Equipment List provided -
  (2) The inoperative instruments and equipment are not -
    (ii) Indicated as required on the aircraft's equipment list, or on the Kinds of Operations Equipment List for the kind of flight operation being conducted;

But the equipment list does indicate the gauge as required, so this exception does not apply.
To fly the airplane to a repair station to fix the vacuum gauge, apply for a Special Flight Permit (or “Ferry Permit”) with your geographic region’s Flight Standards Office, as provided in FAR 21.197.

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text:
Why is jet fuel cheaper than piston fuel?
Why is Jet-A cheaper than 100LL? Shouldn't it be the other way around?

Why should it be the other way around?

Why the heck would it be the other way around? TEL is a specialty additive produced by like two entities in the whole universe, and 100LL is the primary thing it's used for these days (as leaded mogas is only used in a handful of countries any more). Never mind that 100LL is laden with high-octane refinery blendstocks like alkylate that not all refineries can even make; in contrast, Jet-A requires very little additive help, and is not much harder to refine than hardware-store kerosense.

Is Jet-A cheaper than 100LL? Yes, typically it does seem to be cheaper. However, I have seen it both cheaper and more expensive, depending on the location and vendor. See, for example: KSER vs KFRH. Or this classic: KRST

Related: Why is AVGAS so much more expensive than car gas?

Very simple short answer; Jet is cheaper for the refinery to make than high octane gasoline. Pipe stills and cat-crackers (operate a several psi) basically make the jet ; sulfur removal  does add significantly to the cost. To make high octane you need reformers using high pressure hydrogen  (500 psi) at high temperatures  (800 F) .

Because jets are more expensive the fuel should be too.

Jet-A (and all turbine fuels) are far cheaper than 100LL (at least in bulk; retail prices vary due to local supply and demand as well as different regional tax regimes on aviation fuel) for several reasons.
Scale
Southwest Airlines alone burned through 1.9 billion gallons of Jet-A in 2015, dwarfing the mere 175 million gallons of 100LL that the entire US GA piston fleet consumed in the same year.  This means that Jet-A is made and traded in large volumes, enough to merit refiners sometimes optimizing for its production over other products in certain refineries, while avgas is the bastard child of the refining world -- people would rather make other things that are easier to sell than 100LL.
Ease of Production
Jet-A, like all kerosene fraction products, is straightforward to produce: straight-run kerosene that has been run through a few sweetening (impurity removal) processes is mixed with mid-weight distillates from heavy cracking processes (catalytic crackers, hydrocrackers, cokers) and presumably processed further with hydrogen and clay to get rid of more impurities.
On the flip side, there are all of two TEL (tetra-ethyl lead, aka the "lead" in leaded gasoline) producers on this planet that I can find (Innospec in the UK and TDS Chem in China).  Considering that the last few countries that use it in mogas are working on phasing it out, and very few road vehicles sold on the global market these days can even burn it to begin with due to being made to more stringent emissions specs, it'd be safe to say that the majority of TEL made on this planet is going into planes.
Atop that, avgas requires large quantities of high-octane blendstocks, but cannot use any oxygenates (they are prohibited by the avgas spec, aka ASTM D910, as GA fuel systems aren't compatible with 'em, especially ethanol).  These high-octane blendstocks are primarily alkylate and reformate, both of which require specialized refinery equipment to produce.  Furthermore, reformate is an inefficient source of high-octane blendstock (reformate is very heavy in aromatics such as benzene and toluene, which are better off getting distilled out of the stream to be used as a valuable chemical feedstock instead), and alkylation, while an efficient way to produce high-octane blendstocks, requires an acid supply nearby (either sulfuric or hydrofluoric), limiting where alkylation units can go.  Finally, both of these blendstocks are in demand for motor gasoline production as well, which means that most refineries that produce them are filling mogas demand first and avgas demand second.
Handling
Jet-A is relatively easy to handle -- it's a close cousin to the kerosene on hardware-store shelves.  It has a relatively high flashpoint (meaning you need to make a fine mist of it mechanically or heat it aggressively to ignite it), and this low volatility also limits worker exposure to the relatively limited chemical hazards it poses.  Furthermore, it can be shipped readily via most refined-products pipelines without posing unusual hazards or risking excessive cross-contamination by or to other products.
On the other hand, tetra-ethyl lead is highly toxic, and 100LL is toxic not just because of TEL, but because of volatile hydrocarbon toxicity.  While isoparaffins seem to be less problematic than normal paraffins from this standpoint, volatile aromatics (Benzene, Toluene, Ethylbenzene, and Xylene, or BTEX for short) are well known as troublemakers in the human body.
Atop this double handling hazard (volatility of the fuel and toxicity of TEL and light aromatics), pipelining 100LL is a much harder task due to the TEL contaminated transmix (contaminated stuff from the boundaries between products in a multi-product pipeline) it produces.  Normally, transmix is reprocessed by refiners or dedicated transmix reprocessing plants to produce gasoline and diesel (with most of the kerosene fractions in the transmix being lumped in with the diesel for simplicity's sake), but the presence of TEL in transmix renders it unreprocessable as any more than 50mg/gal of lead in motor gasoline means it is contaminated and unsellable as per EPA regulations.  Furthermore, shipping 100LL in a multiproduct pipeline risks contaminating other products with lead batch-wide due to dead legs and other such cross-contamination hazards, which means that in practice, 100LL is never shipped in multiproduct pipelines.
Instead, 100LL is typically railed or barged to terminals, and sometimes even trucked directly from the refinery as not all terminals have rail or barge access.  This drives up shipping costs compared to every other fuel (save for ethanol and other fuel blendstocks that are surfactants).

All them 50¢ words makes you sound like you know what yer talkin' about! :D +1

Why can't the TEL be separated back out of the transmix and the newly-de-TELed transmix used like regular transmix?

@Sean -- it's an added cost item in transmix reprocessing -- TEL and its associated scavengers boil out into a heavy hydrocarbon fraction during distillation (presumably into the diesel fraction?), where it's NFG whatsoever (the radical quenching action of TEL would likely have deleterious effects on diesel engine operation, although nobody seems to talk about TEL contamination of diesel fuel...)

@UnrecognizedFallingObject: NFG?

@Sean No Freakin' Good

There are a number of factors that go into it:
Economies of Scale
Jet-A is used in a much higher volume than 100LL. All the jet engine commercial aircraft each day burn through orders of magnitude more Jet-A than piston aircraft burn through 100LL. That means that there is a much higher production volume, and therefore a cost savings as a result.
Buying in volume
It is pretty rare for a jet to taxi up and take on 50 gallons of Jet-A. Even the smaller jets rarely take on less than a couple hundred gallons. If you went up to your local FBO and said that you wanted 500 gallons of 100LL, they would find a way to get a discount for you.
100LL is special
Leaded fuels are not manufactured anymore for any other purpose than 100LL. There is a push in the aviation industry to do away with the leaded fuels altogether, but this isn't happening for a while. As a result, it takes a special process to add lead to the fuel, equipment not used for any other purpose. Jet-A on the other hand isn't incredibly different from diesel #1 or kerosene, at least in the way it is processed.
Negotiated contracts
Because airlines buy so much fuel, they have negotiated contracts directly with fuel suppliers. These contracts help bring the price down because of competition, and the supplier is running a steady supply of fuel for the airline.
I'm not sure why you would think that 100LL would be cheaper, it's sold in low volume, it is expensive to make, and it is environmentally unfriendly and toxic.

Near as I know there is only one plant left in the US which manufactures tetra ethyl lead.  There can be shortages in North America, especially if AvGas is earmarked for emergency use.

@CarloFelicione -- mind providing a source for that? The only plant I know of in the Western world that produces TEL is Innospec's in the UK...

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text:
Are run-ups required for multiengine piston aircraft?
Are run-ups required or advisable on multiengine piston aircraft?

Yes, From the FAA handbook

...be operated from its crossfeed position during the run-up. The
  engines should be checked individually and allowed to run at moderate
  power (1,500 rpm minimum) for at least 1 minute to ensure that fuel
  flow can be established from the crossfeed source. Upon completion of
  the check, each engine should be operated for at least 1 minute at
  moderate power from the main (takeoff) fuel tanks to reconfirm fuel
  flow prior to takeoff.

And also

Some multiengine airplanes have a ramp weight, which is in excess of
  the maximum takeoff weight. The ramp weight is an allowance for fuel
  that would be burned during taxi and run-up, permitting a takeoff at
  full maximum takeoff weight.

For what its worth a multi-engine piston plane is mechanically double what a single engine plane is, and often carries a similar if not the same engine as a single. The things you are checking in run-up, primarily the ignition system holds true for both single and multi-engine aircraft.

The run-up test is an essential pre-flight test for every piston-powered aircraft. Wether it's single engine, twin engine, or more.
It is important to test if the magnetos work properly and if you have a constant speed propeller, if the governor does what it should and also to renew the oil in the governors system.
You can also see if there are any power problems before you take-off.

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text:
Why does the Boeing 787 have fewer cockpit windows?
Why does the Boeing 787 have fewer cockpit windows? The 787 has only 4 cockpit windows, whereas other airplanes have 6. Here is a comparison between the 787 and 777:

^ 787
V.S.
777 v

Related

That image orientation makes the 777 look huge!

It's an Air France so it might have been carrying loads of croissants and other such baked goods.

No @BobJarvis , sorry, it's KLM ! ;-) AirFrance planes don't have this color scheme (Yes, I did notice it is written "AF/KLM")

@dalearn might have something to do with the 777 being huge,

Of course it's hard to tell with two pictures that may be scaled differently, but it looks like the total surface area is about the same for both.

@dalearn, the 777 IS huge!  777-300ER is 242 feet long, 212 feet wingspan.  This means that if you put one on a football field, with the tailfeathers touching the Dallas goal line, the nose would be 2 feet into the Washington red zone, while the wings extend 26 feet over the sidelines on both sides!

@dalearn: It's like staring down a whale...

It is because of the carbon fibre. This is the same reason that the 787 has bigger passenger windows. Here you can find a link explaining the polarized windows on the 787. Because of its higher tensile strength, you can have bigger windows. Windows are considered weak points in the fuselage where there is no structural reinforcement. This means that the windows have to be supported by the fuselage and structural components around the window. Boeing Pilots do not fly with their elbow out the window during flight, so Boeing removes that feature. They added bigger windows because they could. There is there separation for the six windows because there is support needed.
Hope this helps, Charlie

It looks like it's only me, but I don't see how this answers the question. Unless there is too big of a perspective difference, the photos make it like the 787 has less glass than the 777, and less panels, which is the question. With the reasoning about the stronger structure, I would expect more windows, no less; unless, like I said the picture are from such different angles that they don't show that the 787 has actually more glass than the 777.

@MartinArgerami: You're not alone. Unclear explanation involving the tensile strength (resistance to elongation) of the fuselage and "Boeing Pilots do not fly with their elbow out the window during flight, so Boeing removes that feature". At least the area of the windows and angle of view should be compared.

"fly with elbows out the window" ... Hey, that's a ground feature.

The rules on visibility from the cockpit have been updated.
There are rules and regulations imposed on the view from the cockpit, for very good reasons - collision avoidance for instance. During approach, the pilots must be able to see the runway while on the glide slope, at any COG location. From Torenbeek:

The angle of view forward and downward must be sufficient to allow the pilot to see the approach and/or touchdown zone lights over a distance equal to the distance covered in 3 seconds at the landing speed when the aircraft is

on a glide slope,
at a decision height which places the lowest part of the aircraft at a height of 100 feet above the touchdown zone (see Fig. 3-25) ,
yawing ± 10°,
making an approach with 1,200 feet Runway Visual Range, and
loaded to the most critical weight and center of gravity location.

Some additional requirements from British regulations were:

When taxiing, the pilot should be able to see the ground at a maximum of 130 ft trom the airplane, but preferably this distance should be 50 ft or less.
When climbing, the pilot should be able to see at least 10° below the horizon and preferably 15-20° below it.
When landing, the pilot should be able to see below the horizontal when the airplane is in the tail-down attitude .
Another desirable feature is that during taxiing the pilot should be able to see the wingtip on his side of the airplane.

The FAA made a proposed regulation on improved visibility from the cockpit in 1971. However, according to the aerospace industry, this would impose structural limitations on cockpit design with high weight and drag penalties. From FAA AC 25.773-1, published in 1993:

A majority of the commenters responding to Notice 71-2 objected to the proposed amendments. In general, the airplane manufacturers believed the proposed requirements were too stringent and exceeded the state-of-the-art, particularly with respect to the size of transparent panels, considering weight and structural strength necessary to provide clear vision in the specified areas.

The B777 was designed before 1993 and followed older regulations. The view from the cockpit for aircraft designed after 1993 (the 787 for instance) complies with the Advisory Circular 25.773-1, allowing a smaller field of view compared with earlier regulations.

The windows are curved because current technology allows for double curved surfaces that are strong enough for bird impact, although the B787 has had some problems with cracked windshields.. As @PeterKämpf points out, the flat panels of the past required using more of them to keep reasonable aerodynamics, drag and noise: if the panels must be flat, many small panels make for better aerodynamics than a few big panels.

Thanks for mentioning the curved nature of the newer windows. The flat panels required more sections in the past so the aerodynamics were still acceptable. Cockpit noise should be noticeably lower with curved panels, a clear indication of better aerodynamics.

@PeterKämpf Valid point, have included in the answer.

This should be the accepted answer!

Having many cockpit windows is a relic of past times, where most of the work was VFR and there were no such things as a TCAS.
Let's take a look at the Boeing 707 for example:

Image Source
As you can see, there are many big windows, even additional small ones at the top to make it easy to spot traffic.
However, IFR and systems like TCAS have made it unnecessary to use many big windows. Today, windows in the cockpit are only needed for take-off, landing and taxi. The rest works automatically.
And if you think: "Why bother, just let those windows in there, the better view is always better". Glass is significantly heavier than the other materials used on aircraft. And as you know, every pound counts in aviation.
This change happens slowly as you can see. That's why the 777 has still got quite big windows and the 787, the current Boeing flagship, smaller and less.
This trend will, of course, continue in the future, until the minimum for above-stated purposes is found.

There are minimal standards imposed on the view from the cockpit by CFR 14 Part 25.

Pilots are required to maintain a lookout for traffic even when IFR.  TCAS is not intended to be a substitute for pilots looking out the window.  When flying under IFR rules but under visual conditions, pilots may accept instructions which require them to maintain visual separation from traffic that they have in sight.  In short, the windows are still very much required.

authors: Bob Jarvis - Слава Україні


text:
Would a de-aerating fuel tank have any benefits?
I know turbine engines (bearings) are cooled by oil jets and that when oil is scavenged from the bearings it is more of a mist than a liquid, hence the need for various types of de-aerating this mist to produce oil once again. So is there a need for fuel de-aerating? Especially in fighter jets, because they maneuver a lot, will this cause the fuel to churn in its wing, causing the fuel pump to suck in a lot of air?
As an add on, I read from multiple sources tanks in wings can operate in high-g situations simply by having multiple fuel ports leading out of them. I find this a little confusing. Let's say a fighter is dashing, so all the fuel in its tank is rushing to the rearmost part of the tank. If this tank has a front and a back fuel port, and they are being sucked by the same pump, will this result in something like a 50/50 fuel/air mixture? This has to be dealt with, or the combustor will hiccup, or even worse the flame will back travel into the fuel line and bust it open, am I right?
PS: I know all about baffling, so please don't include this in your answer, thanks!

There are ways of preventing air from being pulled into fuel lines due to change in speed,
direction, or orientation.  On of the simplest is called a "clunk" line, which is a flexible hose with a weighted fuel inlet at the end. The inlet and fuel simply fall to the "lowest" level G forces take it.
As far as fuel entraining air by agitation, important factors are viscosity and the chemical
make up of the fuel.  Fuel is generally "thinner" than oil so air can escape more easily.
Chemically, fuel does not have the tendency to form bubbles as a mixture of soap and water would.  Air entrainment simply is not an issue with flight maneuvers.  No doubt the oil
companies have a good number of chemists to keep it that way.

well then it has more to do with physical attributes of the fuel than any fuel system design, however contrived the latter might be. I wonder if you design a liquid-fueled SAM that can do 40g how do you keep it fueled at all time? If you have ever seen real footage of free air combat war game, you couldn't even find the horizon for most of the time because the fighter maneuver so much everything is blurry solely from motion blur, the g-force is never stable, but constantly randomly pointing at any and all directions.

I guess that is why "fighter manuever" G forces are tough for pilots.  Solid fuel also helps too.

@MeatballPrincess SAMs rarely pull those manoeuvres while the engine is burning, and while it is, the thrust is mostly aligned with the axis of the missile. In a terminal guidance scenario, which is when the lateral loads will spike, the rocket engine will most likely have burned out anyway. Additionally, many (most? I'd have to check) use solid fuel as it is easier to keep on combat readiness.


text:
What is a red airway on the VFR sectional?
The VFR sectionals show a number of airways/routes marked in red. Here are several near KMTH, you can see B-646 extending east of the NDB:

There are others in Alaska that are lettered A, R or G, e.g. A-7 runs between the CMQ and MNL NDBs.
What are these airways or routes and how are they different from Victor airways?

These are Red Federal Airways, and can be found in Order 7400.9
Red airways are one of a 1950s-ish system of airways: Red, Blue, Green, Amber, which are designated by an NDB (rather than a VOR or VORTAC for a V or J route).
Green and Red airways go east/west, and Blue and Amber routes go north/south.
These routes were prioritized: GARB, with Green routes having the highest priority.


text:
What is the average number of flight hours that US private pilots require before passing their checkride?
It's easy to find comments and claims online (including on this site) about the average number of hours that's required for a new US private pilot to pass their checkride. Some are very precise - 67 hours seems popular - but no one appears to have any actual data to support their statements.
This comment is typical:

We aren’t sure that 67 hours is the actual average. You can find as
  low as 55 and as high as 85 as the national average. A quick Google
  search found a dozen different answers to the average flight hours.
  None of the sources seemed authoritative. The FAA doesn’t seem to have
  a published number.

It's quite surprising (to me) that the average number of hours is so mysterious. Checkride applicants have to report their hours in the IACRA application process anyway, so presumably the FAA has the data. And it would be a very useful piece of information for schools and pilots to have for planning/budgeting purposes. In fact, I would expect part 141 schools in particular to have very detailed records on their students' hours, and perhaps even use them as a marketing tool ("get your private certificate in 10hrs less than the national average!").
But despite all that, there's no official, reliable number that I can find. So, what is the average number of flight hours from zero to passing the checkride?
Details:

Number of flight hours required to successfully pass the initial private ASEL checkride
Data from reliable sources only, i.e. FAA or a credible third-party study that identifies its sources and methodology
FAA private ASEL ab initio candidates only (no add-on ratings)
Part 61 and/or 141

This should be fairly easy since all PPL candidates have to fill out the same IACRA form that specifically spells out the number of hours training (and types of hours). I'm not sure if they release IACRA data publicly though. @MichaelKjörling I'm not sure they would be statistically different, CFI's are punished if they push applicants through to DPE's that can't pass. The occasional one is OK, but they can have issues if they push too many through that fail, so they tend not to do that until they are 99% sure.

@RonBeyer I agree that it should be fairly easy to find the number, however as I mentioned in my question it apparently isn't :-)

This document from and published by the FAA does include a range although they do not cite their source or where they pulled the data from. It is as official a number as I can find.

One difference between a part 141 school and a part 61 school is that
  fewer flight hours are required to qualify for a pilot certificate in
  a part 141 certificated school. The requirement for a private pilot
  certificate is 40 hours in a part 61 school and 35 hours in a part 141
  certificated school. This difference may be insignificant for a
  private pilot certificate because the national average indicates most
  pilots require 60 to 75 hours of flight training.

The range most likely varies heavily due to the way people train. I know people who have gotten it in <50 and people who are up around 90 and still have not flown the check ride. It varies largely with how much time you have to train as well as local weather. I was flying once a week (occasionally twice) during my training) and I was somewhere around 63 when I got my ticket.

Good find! It's still quite a wide range, but at least it's an FAA source

Ive added some notes on the range, I think it varies heavily as its quite highly correlated to your abilities to train and how that interacts with your personal life.

@Dave Agreed. I'd think you'd need a lot of statistical data to do any realistic and useful analysis. Off the top of my head, you'd need to know who is doing "full time" training, who can afford it upfront, age etc. FWIW, it took me 60 hours over several years which, sadly, involved often repeating lessons to get back to up to speed (I.e., do something, take a break, do the same thing again)

The averages I have found are what you are seeing, in the 65-hour neighborhood.  You pose some other questions though, and I agree that the numbers should not be as variable.  If you are seeking to obtain your Private license and are trying to cost-out the hours, remember to consider where you are training.  All of your hours are counted from the time the engine starts, so if you are operating out of a busy airport with long taxiways and you are mixed-in with heavy commercial traffic, expect your time to be toward the higher end.  At a busy airport, there are delays waiting for ground control to let you leave the ramp, delays due to the distance to the departure end of really long runways plus you will need to travel farther from the airport to get into suitably uncongested airspace in which you can perform flight maneuvers (then all the same stuff on the way back, the long flight to your home field, delays, long taxi back to the ramp, etc.)  It all depends on what you need the average numbers for.  If time and expense are an issue, students are better off at small airports away from busy hubs.  If PPL student is moving on to a professional career, it's OK if a lot of your time is spent at busy airports, because even if it takes you more time, you are gaining a lot of valuable lessons by operating in/out of the type of airport in which you will find yourself as a professional (and listening to lots of radio communications and learning from them.)  I passed my test on the first try right around the averages you have seen - I was around 65 to 67, fairly busy training airport at a Part 141 school.  As a CFII at a Part 61 FBO, I was sending folks up for check rides closer to 50 hours, so 'average' is heavily dependent upon the size/volume of the airport and the instructor's technique and/or preferences (and the average gets larger if you have large gaps of time in your flying as a student).  But yes, it is odd that it is difficult to find a single average number of hours.


text:
What happened with the YF-16 prototypes?
What happened with the original two prototypes of the YF-16 that were ordered during the Lightweight Fighter Program? Were they converted into combat-capable F-16's?

Some of them are in museums, YF-16 was a technology exploration and didn’t commit to production.

YF-16

72-1567 – Virginia Air and Space Center, Hampton, Virginia
72-1568 – Rome ADC

YF-16A (Full-Scale Development)

75-0745 – Used as a traveling exhibit, on loan from the National Museum of the United States Air Force, Wright-Patterson AFB, Ohio
75-0746 – Pylon-mounted gate guard, McEntire Air National Guard Base, South Carolina
75-0748 – Cadet Area Quadrangle, U.S. Air Force Academy, Colorado
75-0750 – Experimental Aircraft Display Hangar, National Museum of the United States Air Force, Wright-Patterson AFB, Ohio

YF-16B (FSD)

75-0751 – under restoration at the Air Force Flight Center Museum, Edwards AFB, California.
75-0752 – Frontiers of Flight Museum, Dallas, Texas

Why would they put aircraft like these in museums? Wouldn't it have been better to use them as trainer aircraft? These aircraft are expensive, it seems strange (when they were new!) to just commit them to become historical exhibits. Couldn't they be modified to be even combat capable?

@AlphaCentauri It would probably cost more over 25 years to maintain eight unique airframes than to just build the same number of new production F-16s. Unique is bad when it comes to bean-counting.


text:
What does the "remote control" for a QF-4 look like?
As I understand it a QF-4 is a Phantom which is converted to a remotely controlled plane for use in weapons testing.
(Apparently in non-destructive runs, a pilot is still on board for safety, convenience etc, but in actual "really shoot down the QF-4" runs, it is flown as a remotely controlled plane.)
I'd be amazed to see what the remote operator station for a QF-4 looks like.

Is it just a ... RC unit?
Is it just an ordinary PC, perhaps? Fly by mouse/keyboard?
I'm wondering is there a video feed from the pilot viewpoint included with the telemetry?
Is it perhaps flown from another chase plane?
Is it "control control" (the remote operator uses something to actually move the control surfaces, i.e. exactly as actually when flying a plane) or is it "concept control" (you just tell it, go to this point, this altitude etc and the software does that).
Can you land it using the remote system?
Do the operators of these get a chance to make any training flights, or is it just too expensive? The only time you get to take off, get shot at, and perhaps land is during actual runs as a target?

Again, I'd love to see a photo of what the control system/bay/iPhone app whatever looks like!

Footnote.  I believe the QF-4 is retired?  Question applies to a QF-4 replacement!

I hope the USAF has paid for the ad-free version of the iPhone app!

LOl on that one DR !

The air force contracts with government contractors to do stuff  like the QF-4. They are generally forced to chose the lowest bidder that meets the contract. If you were a government contractor who made the lowest bid, and had a bunch of programmers and engineers who grew up on PC's, what do you think you would use?

hey @Sam ! - I'd use a mac.  Actually just an iPad  :)   more seriously we'd use mac or linux (since those are unix) rather than windows, unless, you needed badass graphics cards for some reason, then i'd just use a gaming PC with windows, cuda etc as you say.  I cannot tell in the photo below if those are running Windows!  Which flavor of windows would it be in that era?

Here's a photo:

Retired Lt. Col. Thomas Mudge, a ground controller for the 82nd Aerial Targets Squadron, remotely pilots a QF-4 during a Combat Archer mission May 12, 2015 at Tyndall Air Force Base, Fla.
QF-4s were retired in 2016. They are now flying the QF-16.

Whoa !!   Say, the image on the left under the man's arm: is that a camera (looking downwards at a bay/beach it looks like) or is that just mapping, I can't tell ...   thanks!

@Fattie Most likely it is just satellite imagery. I doubt they have a high-altitude chase plane/drone to follow around and beam real-time images (which would be affected by things like clouds or smog).

@RonBeyer - indeed, what I was wondering (my "point 3") - do these aircraft have a camera, which, feeds to the operator on the ground.  Perhaps answered Gerry knows - it looks like "no" in this example anyway?

@Fattie The QF-4 can fly manned or unmanned. Usually the remote pilot only controls it for landing/take-off and then hands control over to an automated system. The automated system, when flown unmanned, can also blow up the aircraft if it becomes uncontrollable (explosives are loaded for this purpose). There is a chase plane when the QF-4 is transiting to the mission area and back for safety reasons.

Ah so the actual sort of "mission run" is flown by the software on a pre-chosen path eh?  Fascinating.

So does the 82nd Aerial Target Squadron have the highest number of planes shot down? Do the pilots get credit for the number of times they've flown kills? Or for evading incoming fire?

@RobCrawford Sounds like you got a whole new question there.

I don't believe it counts toward "ace" when the plane shot down is yours.

@RobCrawford - apparently it's for testing surface to air missile systems - it's not for combat training.  funny though!

Can anyone tell if that's Windows ?

Hopefully not Windows.  "The QF-4 program attained initial operational capability in 1997."  Back then it was either Win 95 or NT.  Yikes.


text:
What's the precision of a modern Inertial Measurement Unit, and how is it obtained?
This recent comment reports that:

the IMU on new (plane) would localize the aircraft to within 3 feet
  after a cross-country flight, without any GPS input other than the
  starting location.

I somewhat doubt about this statement, at least for an IMU based solely on inertial measurement: over the duration of a flight, I fear much more error accumulates.
So what's the precision of a modern Inertial Measurement Unit over say the duration of a flight, and from what sources is that obtained? If some source (in particular, GPS) becomes unavailable, how does it degrade that accuracy?

related: http://aviation.stackexchange.com/questions/2275/what-is-an-inertial-navigation-system-how-does-it-work?rq=1

For a country the size of Monaco the statement might be true...

the wiki article of INS states (unreffed) that "The inaccuracy of a good-quality navigational system is normally less than 0.6 nautical miles per hour in position and on the order of tenths of a degree per hour in orientation"

@ratchet freak: From my minimal understanding of Inertial Measurement, the error in a purely inertial mode should grow as the square of the time; or at least significantly faster than linearly with time. That makes me doubt of that source, although I think it is much less far off than the bragging I quote.

All INS units that I am aware of have max drift rates specified in NM/Hour so is linear.

@fgrieu Due to the Schuler tuning, the error is only growing quadratically during the first 10 minutes or so. The errors are oscillating with a period of 84 minutes.

Ah, I stand corrected, thanks all. My previous exposure to IMU was in a non-aviation context (robots!) where position is thought for timeframes in minutes.  I found this article which confirms the 0.6 nautical miles per hour.

@fgrieu: I think you should create an answer based on a summary of that link.

Let's have a look at documentation of actual modern IRS. It says (on page 6) that if GPS data is lost, it will maintain RNP0.1 for 8 minutes, RNP0.3 for 20 minutes and RNP1 for 2 hours. That means that in 8 minutes the error is no more than 0.1 nautical mile with 95% confidence, in 20 minutes it is no more than 0.3 nm with 95% confidence and in 2 hours the error is no more than 1 nm again with 95% confidence.
For general description how it works see What is an Inertial Navigation System? How does it work? and of course the appropriate Wikipedia article. Basically it measures acceleration and rotation (with gyroscopes) and integrates it over time. Modern systems do the calculations digitally with sophisticated filters to smooth over some errors.

Those are the certified values. The actual accuracy of the INS is likely significantly better, but the manufacturer can't/won't guarantee that for certification purposes. There's always a safety margin and in systems like this it's usually a pretty large margin.

@dvnrrs: The certification only requires 95% confidence here, which is pretty low. So I suspect the margin does not have to be that large in this case.

@dvnrrs: A safety margin is built into the RNP procedures where the actual clearance from terrain is always higher (I believe at least twice, but am not sure whether it's not more) than the required RNP rating.

In addition to Jan's answer, there are other means than GPS to provide location data to an INS.
VOR and NBD triangulation has been used for decades, long before GPS even existed.
And there's always the good old sextant, which a trained crew member can use to get a pretty accurate position with, in combination with a good clock.

Both can provide location information to update an INS. Of course the update will only be as good as the skill of the crew performing the position calculation.

I am not sure how many IRS are capable of in-flight realignment to those inputs and suspect not many. The older ones certainly can't (I've seen report of an incident where a plane had to land because they switched off INS by mistake and it couldn't be reinitialized in flight).

@JanHudec the 747-200 I know could be updated in flight using VOR data. Not reinitialised, but updated. Initialisation of INS requires the aircraft to be stationary, even strong wind can cause problems in some aircraft.


text:
Why is the use of tail-down force so common?
Why is the use of tail-down force so prevalent in aircraft design?  Why not use canards to avoid induced drag?

Note that even with conventional tail it does not have to produce negative lift. For stability it has to fly with lower angle of attack. But it does not have to be negative.

@JanHudec In common scenarios (GA) for stability it has to produce negative lift: NP is behind CG creating a pitching down moment that you have to counteract with the tail. Positive lift with the tail -> additional pitch down.

@Federico: Not necessarily. At high AoA most tail surfaces have positive lift. Negative lift is a sign of high static stability and low AoA, but you can still have positive stability with positive lift on the tail. Jan is correct, as always. If this needs more explanation, please post a new question!

If it were so simple ...
To achieve natural longitudinal stability (the aircraft stays at the trimmed speed, even after disturbances like gusts), the rear lifting surfaces need to have a lower lift per area than the forward surfaces. Why? In a gust, the aircraft's angle of attack changes on all surfaces nearly at the same time. By having less lift per area, the relative lift increase due to an increase in angle of attack is higher for the rear surfaces, so now they gain relatively more lift increase than the forward surfaces. This creates an imbalance which lifts the tail up, until the old angle of attack (and, consequently, the old speed) has been reached again.
A negative tail loading is a sign of high stability. This creates more drag, but allows the pilot to take the hands off the stick, to read his maps, check something or have a cup of tea. But its effect on induced drag can actually be helpful. You might be surprised, so please bear with me!
What is induced drag, anyway? It is the consequence of creating lift over a limited span. The wing creates lift by deflecting air downwards. This happens gradually over the wing's chord, and creates a reaction force orthogonally to the local speed of air. This means the reaction force is pointing up- and slightly backwards. This backwards component is induced drag! The lengthwise distribution of the surfaces which do the downward deflection is of little importance (for a very technical explanation: See here, where they talk about the Treffz plane): If you have a highly loaded canard wing, its downwash will hit the wing eventually, creating a lot of backward-tilted lift there. The canard by itself will not cause so much induced drag, but will mess up the airflow over the wing. Behind the airplane it does not matter if you fly a canard or a conventional configuration, what matters is over how much span you distribute lift creation (for a constant amount of lift at a given speed).
You probably know that an elliptic distribution of circulation over wingspan produces the smallest induced drag. Now imagine that your wing has more a triangular than an elliptic distribution. The downward lift on the tail will reduce this hump of lift in the middle, making the distribution closer to that ideal elliptic distribution behind the whole airplane. The induced drag of the whole airplane is lowered by the tail!
Another explanation: Since the wing creates a strong downwash at its center, the tail flies in an airflow which points slightly downwards. Negative lift there (being approximately orthogonal to local airflow) will actually point slightly forward, so your negatively loaded tail produces a small amount of thrust!

Black: Total force, Blue: Lift, Red: Drag
And if you think that a triangular lift distribution is unusual, please read this NACA report by R. T. Jones. It takes wing weight into account, and this changes the picture how the lift distribution for minimum induced drag should look.

"You probably know that an elliptic distribution of circulation over wingspan produces the smallest induced drag." Er... Yeah, of course I knew that. Doesn't everyone know that? :P (sarcasm)

@dvnrrs: sioux86 knows the term induced drag. From there it is not far to the elliptic distribution. If he (she? naa, most likely he) has read anything about the Spitfire, he must have come across it.

Consider it a humorous comment noting (and in a way, complimenting) your rather extensive aerodynamic knowledge, and contrasting it with the much more primitive knowledge held by myself and (I have to think) most other non-scientist pilots. Nice answer as usual, but I chuckled at that one line, since your knowledge in that area is way beyond my level.

Yeah, I hesitated briefly, because I used the correct (but uncommon) term "circulation" instead of the usual (but incorrect) "lift". Better to use strange wording than being corrected in a comment, I thought ...

@PeterKämpf, I gave your answer an up vote for all the effort you put into it, but I have to admit, I'm still not getting it.  I still do not understand why it is better to have 2 opposing forces behind the CG, one of which includes the word "down", rather than 1 on each side, both providing lift?  Thanks!

@sioux86: I never claimed that having negative lift on the tail is optimal. It only helps to make the plane more stable, but performance suffers. Glider pilots fly their aircraft in competitions at the most relaxed stability possible, because that gives them the best performance and agility. To be clear: A downforce on the tail is bad for performance, but not a disaster.

@sioux86: Maybe you still think a canard is the better configuration. It is not, even Burt Rutan learned that. Please post a new question, and I will explain what advantages the conventional configuration has over a canard. In my answer I focused more on the highly stable conventional configuration - sorry to have misunderstood your question!

@PeterKämpf, thanks for all the help!  After 6 years, I marked this as answered :/

Yes the first successful fixed wing motorised flight of the Wright brothers used a canard layout, so why did this layout disappear in favour of the conventional tail plane?
Image source
The canard surface disturbs the air stream over the main wing. The long coupled canard places the forward plane at a considerable distance from the main wing, in order to reduce mutual interference. Torenbeek cites the following design issues for this configuration:

To achieve an acceptable range for the center of gravity, the forward plane has to be capable of producing a higher maximum lift coefficient than the main wing. Generally this can only be achieved when the main wing possesses a low aspect ratio. The forward plane has to be provided with a sophisticated flap system.
The trailing vortices of the forward plane affect the flow over the wing and will set up a rolling moment in a sideslip. The vortices may also strike the fin.

The canard layout has as advantage that it provides an aerodynamic nose-up moment with positive lift: it helps with lifting the aeroplane, and if it stalls the nose will come down, auto-correcting the stall situation. The disadvantage of the layout is: at high angles of attack, the canard wing provides a large lifting force, and is always closer to stall than the main wing. That would leave unused a large portion of the lifting capacity of the main wing at low speeds.
So the canard lay-out seems to be particularly suitable for transsonic and supersonic aircraft - conventional aircraft with a horizontal surface at the tail can also cruise with a positive tail load, and have the advantage of a clean flow over the main wing plus a large CoG range.

The idea of putting down force on something you are trying to lift initially does not sound right at all,
but looking closer reveals the true function of the
properly designed H stab, to set the wing at its most
efficient angle of attack while the H stab is at (lowest drag) 0 AOA!
We all know one wing is more efficient than 2.
Really no sense positively or negatively loading
the horizontal stabilizer for optimal cruise efficiency.  The design point at an optimal efficiency
cruise wing AOA would be Clift directly over CG.
The H stab, at zero AOA, would be doing its job holding it there, with sufficient trim and elevator authority to handle other situations.
The added benefit of a rear H stab is it helps directional stability and helps lower the nose if the
aircraft is sinking.
Bob


text:
What are these sensors on the Night Stalkers' Chinook MH-47G?
What are these sensors (or lights) on this heavy assault Chinook MH-47G of the US 160th Special Operations Aviation Regiment, "Night Stalkers"?

Source, picture by Russell Hill
This regiment was the subject of Ridley Scott's Black Hawk Down.

Well... they featured in it but that movie is really about the Rangers.

@Gaius: You're right, the Black Hawks from the 160th SOAR were used to support the Ranger task force.

Those sensors are part of the AN/AVR-2A Laser Warning System. Here is a closeup of the system components.

The AN/AVR-2A is a passive laser warning system which receives, processes and displays threat information resulting from aircraft illumination by laser designators, range finders and beam riding missiles. ... The AN/AVR-2A LDS consists of one interface unit comparator and four identical sensor units.; image from thaimilitaryandasianregion.wordpress.com
These sensors are installed on the MH-47G Chinook, as given in americanspecialops.com

Defensive systems
Common Missile Warning System (CMWS)
Suite Of Integrated Radio Frequency Countermeasures (SIRFC)
AN/AVR-2b Laser Warning System
XM216 Dark Flares (invisible to naked eye)


text:
Why does the IL-76 have a lump at the bottom of its nose?

Why does the IL-76 have this lump at the bottom of its nose?
Why can't it be gotten rid of?

The 'lump' holds a ground surveillance (mapping) radar.
IL-76 has two radars: the usual meteo radar in the nose and the ground surveillance radar (primarily for navigation). In addition to that, like all contemporary Soviet long(er)-range transports (esp. military ones), it has a dedicated navigator position with the glassed 'verandah' for better view of the ground.
Most commonly, this navigator position took up the nose cone space that is (nowadays) usually reserved for the radar. If you look at most modifications of Tu-134 or Tu-114, you'll see they have a similar (but smaller) 'hump' underneath to house the meteo radar. But the widebody IL-76 could afford both the nose radar and the navigator station under it, plus a dedicated mapping radar.
The navigator's seat and equpment don't extend much below the lowest window, so most of the space is used for the radar rather than the navigator.

why don't other aircraft, say like, C-130 doesn't need this bump?

@user366312, well, I guess because they don't have their nose cone occupied by the navigator, and thus can house the radar there.

The lump acts primarily as an extension to the cockpit, as it is known that soviet military planes require more pilots to operate them. The lump is used as a bay to get the navigator a view of the ground.
See (related): Quora: Why-does-the-Ilyshin-Il-76-have-glass-under-the-nose

Despite being equipped with all possible navigational equipment,  there was always the possibility that it all fails (or plane will have to flight in BAD CONDITIONS), in godforsaken places where there is no navigational aids. So there must always be a glass cupola for ground observation, so that navigator could just take a map and give position by visual signs.

Sorry, wrong. See my answer. On the cockpit picture in the quoted Quora answer, you can see two round 'screens' just in front of the seat: one for each radar.

If the navigator were seated in the lump, half of the windows would be above their head. A navigator doesn't need to look at the sky. Instead, the navigator is seated with their head approximately where the top row of windows is to maximize their view of the ground.


text:
I just watched at least 20 airplanes flying northeast over Washington, DC every 15-20 seconds, what were they?
I just watched at least 20 airplanes flying northeast every 15-20 seconds over Washington, DC. They must have been military aircraft since flying very high. I just saw their lights (not blinking). They actually looked like satellites but I doubt so many in the same orbit every 15 seconds.  What is going on?

This is very likely the "train" of satellites from Launch 5 of SpaceX's Starlink constellation. They have about that spacing at the moment, there are 60 of them, and the timing is about right. I saw them myself last night over Nevada.


text:
Do I need AFSP TSA approval once I get my PPL?
I recently got my PPL and wondering if I still require to renew my AFSP TSA approval each year/
I looked into AFSP FAQs but its unclear if I require it again. Any help is appreciated.
Thanks

Merely to exercise the privileges of your PPL, no. Even to take additional training with a CFI for proficiency or endorsements such as complex/tailwheel, no.
To train for an additional rating (e.g. Instrument or Multi-Engine) or certificate (e.g. Airline Transport Pilot, but not Commercial Pilot), yes.


text:
What is the reapplication process for a suspended dispatcher certificate?
I had my dispatcher certificate suspended for a year for leaving the testing site before I could provide an adequate urine sample.
I did not realize this was a reportable event. I accept the suspension but am looking for the process to follow to get reinstated after the 1 year period.

I know very little about this, but JO 2150.3C says that if a certificate is suspended for a fixed period of time, it's reinstated at the end of that time if you meet whatever conditions the FAA requires (p. 7-4):

Indefinite suspensions are used when the Administrator has reason to
  question, but is unable to determine, a certificate holder’s
  qualifications, or when the certificate holder does not comply with
  statutory or regulatory requirements to cooperate with the FAA. For
  indefinite suspensions, the Administrator suspends the certificate
  until certain conditions are met, e.g., until a reexamination or
  reinspection under 49 U.S.C. § 44709(a) is successfully completed. The
  period of time the certificate is not effective is specified in the
  order of suspension, and once the required period has passed, the
  certificate is reinstated.

Your question doesn't have many details - and I understand that you might not want to share them! - but I take that to mean that if your certificate was suspended for one year, it will be reinstated after one year has passed, as long as you've done whatever the FAA wants you to do in order to qualify for reinstatement.
The phrase "until certain conditions are met" implies that there's no standard way to qualify to reinstatement; it probably depends on the reason that the certificate was suspended in the first place. Page 9-15 of the order lists reasons for suspension and they're quite varied, so presumably the "certain conditions" are too.
On the other hand, if a certificate is revoked then it cannot be reinstated (p. 7-4):

Unlike a suspension, a certificate that has been revoked cannot be
  reinstated. A person whose certificate has been revoked may be issued
  a new certificate provided that the person meets the qualification
  requirements for the new certificate. To be issued an airman
  certificate following revocation, an individual must retake all tests,
  whether written, oral, or practical.

Your local FSDO could probably confirm that, although if your situation is more complicated then an aviation lawyer might be more helpful.

Suspensions are not automatically reinstated. Suspensions happen because the person failed to do something or did something wrong, and reinstatements take place after the person has taken some action to mitigate the reason for the suspension, such as counselling, training or passing an examination.

@JuanJimenez Thanks! I've updated my answer

There is no FAR that says your certificate would be suspended for leaving the test site. It is more likely that the suspension was given because your leaving and not returning after being asked to provide the urine sample was interpreted as a refusal to provide the sample. That is grounds for suspension of the certificate. To get it reinstated you need to apply for the certificate following the normal process and take the test(s) again, and don't forget you'll need to provide that urine sample if you are asked for it.


text:
What are the advantages of using a ramjet in an air-to-surface missile?
Which are the advantages of using a ramjet compared to a rocket engine in an air-to-surface missile? I suppose the cost rises a lot, so why do some countries use ramjets as a way of propelling ASM missiles?

According to Wikipedia, only the Soviets used ramjets on a2s missles, because of the combination of long range and high speed.

the French ASMP also uses a ramjet (https://en.wikipedia.org/wiki/Air-Sol_Moyenne_Port%C3%A9e).

A rocket-powered missile carries both fuel and oxidiser and oxidiser is the heavier part. For example a gram of jet fuel needs almost 3.4 grams of oxygen to fully burn! Increasing the fuel load requires also increasing the power, which means even more fuel and this limits the practical range of a missile.
Ramjets are air-breathing, so they don't need to carry the oxidiser, which means the same amount of fuel gives them significantly longer range. Plus the other constituents of air, and extra air, that get pulled along and heated in the engine add to the available reaction mass, further increasing the propulsive efficiency.
Longer range means the aircraft can fire the missile further away and avoid coming into range of the defenses placed around the target, which might make difference between being able to attack a heavily defended target and not, so it is worth the expense to have some in stock.
Also note that ramjet is only more complicated, and expensive, than a solid-fuel rocket motor. Compared to liquid-fuel rocket it is simpler, because it only needs easy-to-handle kerosene while liquid-fuel rockets either need liquid oxygen, or nasty stuff like peroxide or hydrazine. It is also simpler compared to a full turbofan or turbojet (a small solid rocket booster is usually used to get it up to speed).

Very interesting and well-reasoned answer. I wonder why, if ramjet-propelled missiles have so many advantages, are they so rare...

That's because they also have disadvantages. Drag from the intakes, extra complexity, extra handling (fueling the missile before flight), for example.

And lower thrust-to-weight ratio. Unless your missile is meant for very long range, the high TWR of a rocket results in lower total system mass for given delta-V than the lighter fuel of a ramjet.

Jan Hudec’s answer here is pretty good, but the I’d add that it’s more than just oxidizer, it’s the idea with a ramjet that:
1) You can use the air from the atmosphere itself as a working mass.  It’s also the same reason airliners and other airplanes don’t make use of rocket engines for propulsion.  As opposed to a rocket engine which requires the vehicle to pack all the mass used to create thrust from, in a ramjet all that’s needed is the fuel which reacts with oxygen in the atmosphere to heat the air moving through the engine core.  The upshot is that the aircraft has much greater range that it would with a rocket engine, which can only burn for a brief time before going into unpowered ballistic flight and quickly decelerating due to atmospheric drag.
2) Ramjets have no moving parts which make them simple, reliable and light weight as opposed to jet engines or liquid fueled rocket engines.
3) Ramjets are safer than liquid fueled rockets due to not carrying an oxidizer aboard.
4) Ramjets offer good performance at supersonic speeds ie Mach 2 to Mach 5 range, giving the missile a lot of smash or kinetic energy to the target and reducing an enemy’s ability to evade the missile.
The downside to Ramjets is that they won’t work at slower speeds.  This requires another type of propulsion system like a solid fuel rocket booster to accelerate the missile up to its cruise speed before the Ramjet can light and continue the flight.

Would be very interesting to compare top speeds "down on the deck".  The SR-71 was essentially a ram jet at top end, but at very high altitude.

No moving parts - fuel pump?

You can actually make solide fuel ramjets. There are even tricks to make it work as a solid fuel rocket initially until the burning of the grain unclog the air entry, at which point it switches to ramjet mode. This allows to have "built-in" boost stage. This wouldn't be worth it on a reusable engine, but is perfect for a missile.

Simplicity of propulsion translates to cost savings, but there are tradeoffs in using a ramjet in a missile. The reason most aircraft-carried missiles do not use ramjet engines is because such a missile has to be accelerated to a sufficient speed for the engine to begin producing thrust.
"A ramjet equipped aircraft requires another type of propulsion to accelerate it to a speed at which the ramjet is capable of producing thrust. A ramjet can theoretically be started at speeds as low as 100 knots but it does not start to produce any significant thrust until the airspeed reaches approximately mach 0.5. Even at this speed, efficiency is very low and peak efficiency will not be attained until reaching supersonic speeds in the realm of mach 3."
What this means is that such a missile can only be carried by aircraft that can fly at mach 0.5 or above, or they have to be equipped with some other means of propulsion to accelerate the missile so the ramjet can start operating. It is much simpler to equip it with a rocket or turbine engine, depending on the requirements of the design.

It's a slightly odd question, as for the most part a ramjet isn't an alternative to a rocket engine for an ASM, but an alternative to a turbojet or turbofan engine.
On the whole, rockets are normally used in short-range weapons (typically on semi-ballistic trajectories), whereas cruise missiles (utilising aerodynamic lift for flight) use various types of jets for propulsion. A few older cruise missiles (particularly in the anti-ship arena) use rockets, such as the Styx/Silkworm missiles and the older Exocets.
For a long-range cruise missile ramjet propulsion provides the cost/benefit split versus other jets as you'd expect - more fuel means a larger missile, but a higher speed makes interception harder. Flying faster also makes for more difficult guidance (read expense), and the extra heat (along with possibly having to fly higher) negates some of the interception advantages. There's a distinct East-West split on whether this is worthwhile - Soviet/Russian weapons tend towards large and very fast (using ramjets), whereas Western missiles tend to be both smaller and cheaper, and avoid interception by means of stealth and increased numbers launched.

The advantage is that your air to surface missile can be supersonic and run on cheap gasoline. The Air Force BOMARC was surface to air ram jet powered, capable of speeds greater than Mach 2. They were accelerated to operational speed with simple solid fuel boosters. These could easily be converted to air to surface, launched from an aircraft
travelling at sufficient speed.
But alas, surface to air missiles are much faster, and an incoming supersonic, although much faster than a slow stealth, will be detected much sooner.
However ramjets may make a comeback if they are applied to supersonic cargo or passenger aircraft.  Marquardt, if they are still around, might be the ones to call.


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How to understand the requirements for IFR currency?
So the requirement reads "six instrument approaches;
Holding procedures; and
Intercepting and tracking courses through the use of navigation systems."
I am still a little unclear on how to interpret that statement. I have been told you need to have a combination of the above that equal 6, but you could also read that as you need 6 approaches, 6 holds, 6 Intercepting and Tracking through Navigation Systems. Can someone help explain what I am missing from that rule?

The regulation here seems to be 14 CFR 61.57(c)(1). The way that the regulation is written, we can see that there are three separate requirements:

(1) Use of an airplane, powered-lift, helicopter, or airship for maintaining instrument experience. Within the 6 calendar months preceding the month of the flight, that person performed and logged at least the following tasks and iterations in an airplane, powered-lift, helicopter, or airship, as appropriate, for the instrument rating privileges to be maintained in actual weather conditions, or under simulated conditions using a view-limiting device that involves having performed the following -
(i) Six instrument approaches.
(ii) Holding procedures and tasks.
(iii) Intercepting and tracking courses through the use of navigational electronic systems.

So it's not "a combination of the above that total 6", and it's not 6 of each. It's 6 of the first one, and any amount of both of the other two.


text:
What is the difference between an aileron and a flap?
I have already been searching Google for the difference between an aileron and a flap, but I am not much satisfied, so now I am asking here on SE.
What is the difference between an aileron and flap? Especially, how do they differ in their shape and position?

Just to confuse things a bit, some planes have "flaperons", that combine the functions of flaps and ailerons.

Really, @Man? I googled flap vs aileron and it came up with this detailed Wikipedia page.  https://en.wikipedia.org/wiki/Flight_control_surfaces

Zero research effort.  What is the difference between a wheel and a tail?

An Aileron is used to control the roll of an aircraft. Ailerons are found on the trailing edge of the wing, typically closer to the wing tip.
Ailerons will move in opposite directions to each other, as one goes up, the other goes down.
Flaps are used to increase the amount of lift that a wing produces by increasing the camber and surface area of the wing. Typically they are located near the root of the wing. Flaps on each wing are moved together, both extend and retract at the same rate.
Flaps are generally more complex depending on type. The basic flaps will just pivot down from the lower edge of the wing. Modern airliners have a fowler flap which both pivots down and extends back from the lower edge of the wing.


text:
Does the AOA and G-Force increase if a fighter uses the afterburner during a max G level turn?
My understanding is that if you apply power (increase the thrust) for a jet in level flight, it will momentarily pitch up, and then gain some speed.
So, if a fighter pilot lights their afterburner at a certain time when in a max-G level turn, does this further increase the achievable angle of attack and Gs, due to the increased thrust from afterburners helping to pitch the aircraft up, or does it just allow the max G-force to be sustained for longer?

I'm having a really hard time understanding this question. Can you edit it so that it is more comprehensible?

My understanding is that if you apply power (increase the thrust) for jet engine aircraft ,(in level flight, and some AOA ~11, it will momentarily pitch up to the ~14AOA, due to the additional power , so if afterburner is open then it will be possible to get the more AOA /G's or just sustain the G's already have it before doing this power increase?

this did not help at all. is your question if the aircraft exceeds it max G if the afterburners are switched on while turning?

The way I see it it work is  that, the increase in AOA with afterburner on , will add drag so will not be  "significant"G 's just  an number of  @0.5G. and keeping it longer than without afterburner.My question, is this is true (possible) ?

Lighting the afterburner will only cause a pitch change and AOA change if the thrust line of the engines does not pass though the Center of Mass of the aircraft. If the thrust line pases above the center mass, increasing thrust will cause a nose-down pitch change, if the thrust line is below the center mass it will cause a nose up pitch change.
But, more significant, is the angle of the aircraft thrust vector with respect to the aircraft's velocity vector. Since you posit that the aircraft is in a high G turn, that means that the thrust line is definitely tilted with respect to the aircraft's velocity vector, so increasing the thrust will increase the force turning the aircraft, by an amount = to the delta in thrust times the sin of the angle between the thrust vector and the flight path (velocity) vector.
This might momentarily increase the g-force, but since you posit that the aircraft si already at Maximum G, the pilot would ease off on AOA to let the G-force settle back to the aircraft's maximum G. What the increased thrust would allow, is performing the turn at the aircraft' placard maximum load factor (G) at a lower airspeed than would be possible without the increased thrust...

That is the way I believed   it will the jets what  have afterburner benefit from used in the mix of turning fight Although it is possible for the pilot to not unload at all the G's he /she pulled because of the P/S  at the time. Thanksfor your time.

TL;DR: Maybe.  Maybe not.  It depends on the particular fighter jet, and how it's loaded, and the effect will, in any case, be very small.
An increase in thrust causes an airplane to pitch up via two mechanisms (more details in this PDF):1

An increase in thrust causes an increase in speed; for airplanes with positive speed stability (which is true for all civil aircraft, but is often not true for newer fighter jets), this causes the airplane to pitch up and slow down.2  This changes the slope of the airplane's flightpath (making it point more up), but (at least at first approximation3) does not change its angle of attack.
If the thrust from the airplane's engines is not exactly aligned vertically with the airplane's center of mass, an increase in thrust will create a change in the airplane's pitching moment, which does change the airplane's attack angle; if the engines apply their thrust below the airplane's CoM (examples: 737, B-58), increasing thrust will pitch the airplane up to a higher AoA, whereas, if the engine thrust is applied above the CoM (examples: DC-9, Learjet 23) increasing thrust will pitch the airplane down, decreasing the AoA.  Most fighter jets have engines that are very close to the vertical location of the CoM, so the change in pitching moment from a thrust increase is very small, and its exact direction and magnitude will depend strongly on the specific aircraft and loading used (for instance, burning off fuel could change the vertical location of the airplane's center of mass - and, thus, the magnitude, and, possibly, even the direction, of the change in pitching moment caused by a thrust increase).

Thus, going from maximum dry thrust (no afterburners) to maximum thrust (with afterburners) could, depending on the aircraft, produce a slight increase in angle of attack and vertical4 load factor, but:

the effect would, at best, be quite small, and would depend strongly on the specifics of the aircraft and its loading;
with certain aircraft and/or under certain loading conditions, this effect could well be negative (in which case engaging afterburner would decrease the maximum achievable AoA and vertical load factor); and
the main effect would be (as you, in the latter part of your question, surmised could possibly be the case) to allow a high-G manoeuvre to be sustained for longer (due to the increased thrust helping to counter the high drag of a high-attack-angle manoeuvre).

1: For the purposes of this discussion, I am ignoring aircraft with thrust-vectoring capability.
2: Most airplanes will initially enter cyclic phugoid oscillations with anything other than a very slow, gradual thrust change, but the phugoid eventually damps out, and, even during the phugoid cycles, the "constant-attack-angle" part of the situation remains approximately true.
3: In practice, an airplane undergoing phugoid oscillations with a non-infinite period will undergo continual small changes in its attack angle, due to various damping effects, but the AoA does remain approximately constant.
4: Vertical from the aircraft's perspective, that is.

Increasing thrust increases specific power, but it does not guarantee that Excess Specific Power is positive. Since op posits that the aircraft is at Maximum G, it is quite likely it is operating at or near corner velocity, where excess specific power may be very negative, and the aircraft may (dynamically) be rapidly decelerating. Lighting the afterburner in this case will not cause airspeed to increase, it will only slow the rate of decrease. So the effect on pitch due to airspeed may be completely the opposite.

Also, by definition, if the aircraft is at maximum G, it is at or above corner velocity, where increases in speed (if maintaining Maximum G), will cause a decrease in AOA and pitch attitude.


text:
How to safely handle AVGAS (100LL)?
Obviously 100LL is risky. It's got lead in it, so there's going to be some danger. But is it possible to not get lead into my system when I start flying?
It is, for obvious reasons, something I wish to avoid. I want to learn to fly, but after reading the SDS for 100LL, I'm pretty worried. Here are the hazard and precaution statements for 100LL:

H225 Highly flammable liquid and vapour.

H304 May be fatal if swallowed and enters airways.

H315 Causes skin irritation.

H336 May cause drowsiness or dizziness.

H361d Suspected of damaging the unborn child.

H373 May cause damage to organs through prolonged or repeated exposure through
inhalation.

H411 Toxic to aquatic life with long-lasting effects.

P202 Do not handle until all safety precautions have been read and understood.

P210 Keep away from heat, hot surfaces, sparks, open flames and other ignition sources.
No smoking.

P260 Do not breathe mist/vapours.

P273 Avoid release to the environment.

P280 Wear protective gloves/protective clothing/eye protection.

P301+P310 IF SWALLOWED: Immediately call a POISON CENTER or doctor/physician.

P331 Do NOT induce vomiting.

P303+P361+P353 IF ON SKIN (or hair): Take off immediately all contaminated clothing. Rinse skin
with water/shower.

P308+P313 IF exposed or concerned: Get medical advice/attention.

Some of that is common sense, but that's a lot of very concerning information. How can I avoid contact? It seems like there's not really a good way around it, but I'm really concerned and I want to balance minimizing risk with still being able to fly.
Can anyone offer some advice?

These hazard and precaution statements don't really sound all that dangerous to me. It basically says don't drink it, don't shower with it, don't sniff it.

The ones that are most concerning to me are "P303+P361+P353" and H373. You make a fair point, but I don't think it's something to be taken particularly lightly.

How different is this from the warnings for the 87 octane gas we pump into our cars routinely?

If you look at any hydrocarbon fuel you'll see very similar warnings, including unleaded gasoline. Avgas has some more aromatic hydrocarbons than regular mogas but you still wouldn't sniff or drink mogas.

Fun fact: gasoline was originally a byproduct of kerosene manufacture that oil companies didn't know what to do with. One of the things they marketed it as was a hair tonic!

Lead can be absorbed through consumption and breathing, but generally not absorbed through the skin. If you're breathing avgas or drinking it, you have bigger problems. Also, the body sheds lead. Lead poisoning occurs when you consume more lead than the body can shed.

Everything in those warnings relates to the gasoline, not to the lead content of 100LL.  They almost certainly would be exactly the same for ordinary automotive gasoline.  As has been said, the poison is in the dose.  Certainly anyone who has done a reasonable amount of auto mechanics over the years has had plenty of contact with small amounts of gasoline, with no ill effect.

I might say, this question is strangely "unscientific" coming from someone who wants to be a pilot!    The car petrol at your local gas station has the identical warning bumpf.

I didn't realize that the warnings were the same as mogas! That's surprising, considering the lead content. Also, had no idea the body can shed lead. (That rhymes.) All good information.

@MD88Fan My understanding of lead poisoning is that it's mostly just harmful to children. Even then, though, the doses from being around avgas aren't likely to be sufficient to be measurably harmful. The problems from lead were mostly from back when we had lead in pipes, paint, and car gas. I'm only 35 and even my generation had much, much higher average blood lead levels than the current generation. For all the stink that was made about Flint, the highest average blood lead concentration numbers I saw from there were less than the national average when we millennials were kids.

In order to be more understandable from an international audience, could you edit question and define SDS (or a link?), thanks.

Interesting factoid (maybe). Pompeii had indoor plumbing prior to the eruption  of Mount Vesuvius. The pipes were made of lead. Maybe, we should add that to the warnings.  . If you combine cooking oil, alcohol, water, and an onion the right way, you can also get a volcanic eruption. Where are the warnings for that.

If you don't crawl underneath and breathe in the exhaust fumes from the engine, you'll be fine.  If you do crawl underneath and breathe in the exhaust fumes, you'll be dead from carbon monoxide poisoning long before any harmful effects from the tetraethyl lead combustion byproducts in the fuel.
While you don't want to ingest lots of lead, that won't really happen being around airplanes with engines running.  The old saying in toxicology is, "The poison is the dose", and whatever dose you get from casual exposure is too microscopic to matter.
My advice, as an old-timer who likes a clean world as much as the next guy, is don't become so hyper-sensitized (by mostly media coverage) to environmental exposure to chemicals that you can't enjoy life. It's not worth it.

Those both make sense. The part that really worries me the most is skin exposure, probably from dripping fuel since wearing gloves for fueling seems like a good plan.

I've never been concerned about getting avgas on me - again, the poison is the dose - but on the other hand, it's pretty stinky, and the smell is hard to get rid of, so wearing nitrile gloves while draining sumps, if only to keep the smell off your hands, makes sense in any case.

There is a non-trivial risk of getting avgas on your hands while fueling or sumping, but just wash your hands ASAP, and in particular don’t touch anything you’ll eat or drink until you do.

I always get some on my hands when sumping, but I use car gas which doesn't seem to smell at all once it fully evaporates.

@StephenS makes sense! The one issue I see with that is A) I didn't really think you went back into the FBO after preflight but that's not critical and B) what about airports without bathrooms (yeah, odds are low that they have avgas if they don't have restrooms but it's not inconceivable)

@MD88Fan Worst case, keep a pack of wet wipes in your flight bag, in an resealable bag.

@StephenS Granted, while those do help somewhat for cleaning stuff off of your hands, their primary purpose is killing/deactivating germs. The alcohol is very effective at killing germs, but not so effective at removing avgas from your skin. Better than nothing, of course, but not as good as actually washing your hands.

The poison is the dose but lead is a cumulative poison. The dose is today's nanogram and yesterday's nanogram and all the nanograms absorbed over a lifetime. Over fifty years that would be 18 micrograms. Since (I just looked it up) 14micrograms/dL of blood is not alarming in a child, I wouldn't worry about it.


text:
Why does the Harrier jet have four landing gears?
I was looking at pictures of Harrier jets and I noticed something — they have four landing gears.

That seems pretty unusual for a fighter jet, as most have only 3 landing gears in a triangle formation. What is the reason for this 4-point configuration? It seems to just add complexity and cost without really increasing stability.

I'm going to go out on a limb and say that a standard tricycle gear setup would have put the main gear directly in the blast zone of the vectorable exhaust. Having the 2 main gear on the center line keeps them out of all exhaust zones, and the little outriggers keep it from tipping over.

As well, there is a difference between standard airplanes and harrier, it lands vertically so we can't compare both.

@TrebiaProject. it can also take-off and land conventionally so the comparison remains valid.

@ratchetfreak maybe we can compare as well with Piaggio p136 and CL415 they also take-off and land convetionally. The point I am making is that there is an stronger requirement for design (vertical land) changing the concept. Applicable as well to amphibious planes.

The main load bearing landing gear is in a bicycle setup, with 2 outriggers for stability.
This allows the gear to be lighter, stay close to the center and out of the way for the jet blast.
The Lockheed U-2 also has its gear in the bicycle configuration and takes off and taxies with detachable "pogo" outriggers for stability.

The Harrier is designed around its rather large centerline engine with its 4 non-standard exhaust ports. In order to balance properly, the engine's nozzles have to be in just the right place and we can't have annoyances like landing gear doors opening just before landing and deflecting the thrust stream that is keeping the thing in the air.
Fighter jet wings are typically too thin to fold the gear into them like on some airliners (and the bombs / missiles get in the way too).
So, the gear gets to fold up into the space just behind the aft nozzles where it doesn't disturb anything important. In almost every other jet fighter the engine occupies this space instead. Yes, tricycle gear makes for easier landings but as this plane is designed to land straight down that isn't a big issue. And we tend not to put green pilots into the things either so if they do need a regular runway the driver should be fairly good.

This was the lowest mass solution at the time of P1127 development.
Remember that weight is at a premium for vertical take off. The first hover tests of the P1127 prototype were only possible with fuel for only 3 minutes in the tanks and 700 lbs of equipment stripped from the airframe. A conventional landing gear would had made hovering impossible in 1960.
But be warned that this gear layout will not be easy to get right. It took until 1967 to get it to behave well. What went wrong?

In the first tests, the main gear oleo strut had more travel than the outriggers, so the airplane would tilt over to one side as soon as one of the outriggers would be lifted from the ground. The sideways component of thrust thus set free would let the plane skid sideways, which in turn resulted in a rotation around the nose gear. With thrust being equal to weight, even 6° of tilt will produce a sideways acceleration of 0.1 g!
So for the next test the outriggers were placed on platforms. While lift off was possible this way, landing inevitably took place without those platforms, and the same dramatic sequence unfolded at all subsequent takeoffs. Bleed air was at a premium, so roll control was insufficient to keep the airplane level.
The first successful hovers were possible after the outriggers were fitted with temporary extensions. Now it was time to try a conventional takeoff, starting with taxi tests. The landing gear fought back hard by exhibiting a large deadband in the nosewheel steering mechanism, followed by far too great sensitivity once control inputs exceeded this deadband. The resulting weaving made takeoffs impossible.
When that was fixed, the main wheel brakes took over in prohibiting takeoffs. During the next taxi tests, a brake induced torsional oscillation developed and broke a main leg.
After that it was the turn of the outriggers to act up. They were initially castoring, but showed a tendency to shimmy and finally had to be locked for flight.

Only in May 1961 was the first conventional take-off possible. The nosewheel steering system was barely useable, the brake judder was severe, the anti-skid often failed and the pilot could not feel that the wheels were skidding at any time. Several tyres were burst.
Source: John Farley's Harrier Development lecture.

RE "The sideways component of thrust thus set free would let the plane skid sideway" -- just curious-- not exactly following-- how does tilting (banking) create a sideways component of thrust?  Are we really talking about a sideway component of lift?

@quietflyer This was in hover, and thrust acts in the airplane coordinate system. Tilt the system and thrust tilts sideways, too.

And if the Harrier had been a conventional terrestrial fighter, it probably would have used a ubiquitous tricycle landing gear.  The driver for Harrier's unusual landing gear configuration is the requirement to hover via an unconventional single engine vs available fuselage space.  This necessitated that the Rolls Royce Pegasus engine to be located directly over the center of gravity for the aircraft.  Consequently, this dictates the placement of the engine right on a/c centerline about midway along the fuselage and the engine now occupies much of the fuselage space which could normally be used for stowage of a tricycle landing gear when retracted.   The rotating engine nozzles and their associated exhaust also would interfere with a tricycle main gear located on the fuselage.  Designers both at Hawker Siddley in the 1960s and McDonnell Douglas in the 1980s looked at the problem and concluded that a bicycle type landing gear mounted fore and aft of the engine with retractable outrigger gears on each wing represented the best solution in regards to weight and balance, structure, and drag for the aircraft.

…  their associated exhaust also would interfere with a tricycle main gear located on the fuselage exactly! This was one of the few unsolved problems on the VJ 101.